It's family party time, celebrating a Big Birthday; and all hands to the pumps. Would everyone rally round and help a bit, asks one of my daughters, the hostess with mostest responsibility. At which point the F-word comes into its own. Will you please lay the table for lunch, she asks her elder daughter, nine. "But that's not fair," comes the instant reply. "I helped yesterday." So perhaps her younger daughter, eight, could take a turn with the knives and forks? Alas, that doesn't seem fair either. Long-suffering granny volunteers herself for table-laying duty.

And it's precisely the same when the meal is over. Clear those plates away? "Not fair," says nine, implacable in her sense of injustice. Why can't their brother, 11, do it? "Not fair," echoes eight, never knowingly outbid in the chores rejection stakes. Eventually, protesting, they carry away a dish apiece while their dad does the rest.

They don't seem – you know – over-eager on the domestic front, I say to my grown-up twin girls as we sit and postprandially chat. What do you mean, they cry, rising together in synchronised wrath. We remember how you and mum always made us clear up while the boys (their two older brothers) did nothing. You were just so sexist about it, you know. Washing up was woman's work to you. It just wasn't fair.

Now, neither mum nor I quite recognise this version of long-ago life – especially its facility in writing our younger son, a tirelessly concerned helpmate and guardian of those little girls, right out of the record books. But I can't get the twins to acknowledge that. They were blighted by unfeeling unfairness in their own eyes.

Such exchanges misrepresent a far sunnier birthday weekend, a 99% happy time. But at the party itself I meet a nice local government manager who, wage freezes apart, has just had his pay packet shrunken by 5%. The nation's in a terrible mess, he was told. Here's what you can do to help. Except, he says bitterly, that his bosses – who could afford to help a damned sight more – have just imposed a bog-standard cut when there are whole, useless departments they could have axed entirely. Cue inevitable chorus: "It's absolutely not fair."

And that – from table-laying to white-collar administering – is the great pending problem with the mantra from the weeks before the 6 May election. "Fair" can be a bland, cuddly concept, one to snuggle alongside "change" – as in the Lib Dems' Change That Works for You: Building a Fairer Britain. We all like a bit of something different from time to time. We all think we deserve our share. But pieties go out of the window fast when crunch turns to push turns to shove.

In coalesced government, celebrated by a merged manifesto of excruciating length, the Liberals now offer the total contradiction of "freedom, fairness and responsibility", while Labour, pondering the wreckage of A Future Fair for All, has concluded that it wasn't (for them, that is). And the old saw that you can't prove a negative appears to be totally wrong.

In F-World, it's the positive that doesn't work for anyone. Nobody ever wakes up in the morning and says: I spy a fairer Britain. Nobody, at the end of a dispute, hails a wonderfully fair outcome. BA cabin crews won't praise Willie Walsh's (eventual) fair deal. Bank customers won't define a fair bonus.

The truth is that fairness is a mush word, a fudge concept, a useless political bromide – and only the negative works. My daughters remember what wasn't fair (to them). My grand-daughters find nothing fair about washing-up machines. My party guest – spitting blood – knew an affront when it came off his pay slip. And we too, I rather fear, will soon discover what the real F-word means. Just F for effing unfair.